The invention relates to the field of optical modulator, and in particular to a method of modulating light in SiGe utilizing the Franz-Keldysh (FK) effect.
The FK effect states that the optical absorption of light in energy near the bandgap can be changed by the presence of an electric field. This effect has been utilized in direct gap systems, such as GaAs, to create electro-optic modulators, because the absorption coefficient changes very rapidly in the vicinity of the band edge. However, because phonons must be involved in absorption in an indirect transition, absorption near the band edge is very weak in indirect semiconductors. Weak absorption makes them poor candidates for optical modulation. For this reason, the FK effect has not been effectively used to create electro-optic modulators in indirect semiconductors, such as Si or Ge.
Although Ge is an indirect semiconductor with a bandgap of approximately 0.68 eV, the direct transition lies at 0.8, which corresponds to 1.55 um light. Ge is highly absorptive at 1.55 μm, so it cannot be used to make a modulator. However, the addition of Si raises the direction transition energy, creating a material with relatively low absorption.
The FK effect has been used in the InGaAs materials system to make linear electro-absorption modulators. However, this effect has not been utilized in any Si-compatible materials system to make electro-optic modulators. Furthermore, the FK effect has not been used before in any system to make modulators utilizing a resonant structure or interference effect. Both of these applications are described in this invention.